Buck converters are switching voltage regulators that operate in a step down method to provide a voltage output that is smaller than the input voltage. It accomplishes this by causing the circuit topology to change by virtue of turning on and off semiconductor devices. It uses signal switching to transfer energies into inductors. It uses a low pass filter scheme to eliminate high frequency harmonics to maintain a relatively constant output voltage and reduce the ripple of the output.
Typically buck converters use a feedback circuit to regulate the output voltage in the presence of load changes. They are more efficient at the cost of additional components and complexity.
Buck converters can be made very compact. Therefore they are popularly used for mobile devices, printed circuit boards, even in integrated circuit packages.
An example of a prior art buck converter circuit 600 is illustrated in a circuit schematic block diagram in FIG. 6a. The circuit 600 includes the a P type switch SW1 612, an N type switch SW2 614, an energy storage inductor L 630, the low pass filter capacitor CF 632, and the output load RL 634. The input voltage Vin 610 is the given high voltage. The output voltage Vout 636 is the converted voltage that is usually lower than Vin 610. The Vcom 640 is the common reference ground of the buck converter circuit 600. The control voltage VGS1 616 is coupled to the gate of the switch SW1 612 to control its on and off status. The control voltage VGS2 618 is coupled to the gate of the switch SW2 614 to control its on and off status.
The control voltage VGS1 616 and VGS2 618 are complementary to each other. It means that when VGS1 616 turns on the switch SW1 612, the VGS2 618 turns off the switch SW2 614. When VGS1 616 turns off the switch SW1 612, the VGS2 618 turns one the switch SW2 614. Hence, there are two working states for the buck converter in one working cycle.
One example of the working cycle is illustrated in FIG. 6b. During the time period Ts1 662, the switch SW1 612 is turned on. Then the switch SW2 614 is turned off. The node voltage Vs 620 is equal to the input voltage Vin 610 since the SW1 612 is on with almost zero voltage drop. Then the buck converter storing the magnetic energy into the inductive coil charges inductor L 630. Then during the time period Ts2 663, the switch SW1 612 is turned off Base on the Lenz's law, the inductor L 630 will instantaneously maintain the current flowing through it. Therefor the loop current will go through the switch SW2 614 and turn it on. The node voltage Vs 620 is shorted to the common reference Vcom 640. The total working cycle is Tsw 666. It is obvious that Tsw=Ts1+Ts2. The inductor and the capacitor form the low pass filter that filters out the high frequency harmonics reaches the output Vout 636. As a result, the output voltage Vout 636 is relatively constant with very small ripples.
If the ratio of Ts1 over Tsw is defined as the duty cycle D, D=Ts1/Tsw. During the period Ts1 662, the current through the inductor 630 L ramps up linearly. During the period Ts2 664, the current through the inductor 630 L ramps down linearly. To make sure the ending current of the ramping down is equal to the starting current of the ramping up so that the buck converter 600 can maintain balance, the ratio of the output Vout 636 to the input voltage Vin 610 must be equal to the duty cycle:Vout/Vin=D 
The output voltage Vout 636 can be further controlled through feedback schemes. One popular method is the pulse width modulation (PWM) method. The PWM mode operates switches in synchronization with a clock that has a predetermined cycle. The magnetic energy stored in the inductor is repeatedly increased and decreased periodically. Hence, the power is transferred from the input Vin 610 to the output Vout 636. The output can be stabilized to a desired level by turning on and off the switch during synchronization with the clock. This mode is optimal for mid and high load current. However, it is not very efficient at lower load currents. Then if a buck converter 600 is to operate efficiently over a relatively wide range of load currents, including low load currents, the pulse frequency modulation (PFM) will be used.
The PFM is similar to PWM in the sense that the switch SW1 612 can be used to produce a series of inductor current pulses that are applied to the filter capacitor CF 632. However, the frequency of the pulses is not fixed. It varies in order to maintain a regulated output voltage between the upper regulated output voltage level and the lower regulated voltage level. At low load currents, PFM can provide increased efficiency as compared to PWM for the same current output. This is particularly true since the PWM operation has been optimized for efficient mid and high load current operation.
In view of the foregoing, buck converters have been designed to operate in a PWM mode for mid and high load currents and PFM for low load currents.
It is commonly known that a smaller inductor L 630 in the buck converter 600 give faster transient response, and the larger inductors give higher efficiency. High efficiency is important in all modes. But the link between the inductance of the inductor L 630 and the efficiency is particularly critical in the pulsed frequency modulation (PFM) mode. Fast load transient response on the other hand is most important in modes optimized for high load currents that can be handled by the pulse width modulation modes (PWM).
Inductors are just becoming available where several coils are embedded in the same package. These differ from those previously available in that their coupling ratio is much lower. Traditional multi-inductor packages were designed for use as transformers. Therefore they have a coupling ratio approaching 100%. However, with the new manufacturing techniques, multi-inductor packages are available where the coupling ratio is around 10%.